Cryptology ePrint Archive: Report 2013/728

Modified Alternating Step Generators

Robert Wicik and Tomasz Rachwalik

Abstract: Irregular clocking of feedback shift registers is a popular technique to improve parameters of keystream generators in stream ciphers. Another technique is to implement nonlinear functions. We join these techniques and propose Modified Alternating Step Generators built with linear and nonlinear feedback shift registers. Adequate nonlinear Boolean functions are used as feedbacks or as filtering functions of shift registers in order to increase complexity of sequences produced by individual registers and the whole generator. We investigate basic parameters of proposed keystream generators, such as period, linear complexity and randomness.

Note: Having nonlinear, maximal period registers, we tried to apply them in keystream generator - the ASG proposed by C. GŁnther. But NLFSRs we obtained are to short, so we propose other modified ASGs with nonlinear filtering, combining functions. Some properties of these generators were investigated in this paper. More work is needed to analyze their resistance to the cryptanalysis.